/Engine/Source/Runtime/SlateCore/Public/Widgets/SWidget.h
Called when a child is removed from the tree parent's widget tree either by removing it from a slot. This can also be called manually if you've got some non-slot based what of no longer reporting children An example of a widget that needs manual calling isSWidgetSwitcher. It keeps all its children but only arranges and paints a single "active" one. Once a child becomes inactive, its cached data should be removed.
void InvalidateChildRemovedFromTree ( [SWidget](API\Runtime\SlateCore\Widgets\SWidget) & Child )